The Pentagon says an Iranian fighter jet approached a U.S. surveillance drone over the Persian Gulf.

Spokesman George Little said the incident occurred on March 12 over international waters.

He said an Iranian F-4 plane came as close as 25 kilometers to the unarmed MQ-1 Predator drone, but flew away after the pilot of a U.S. escort plane radioed a verbal warning.

Officials said no shots were fired.

The Pentagon initially said one of two U.S. escort aircraft discharged a flare as a warning to the Iranian plane, but officials later said no flare was let off.

Little said the U.S. drone was conducting a "routine classified" surveillance mission.

It was the latest of several incidents involving U.S. drones and Iran.

Last November, an Iranian fighter jet fired upon, but did not hit, an unarmed U.S. drone over the Persian Gulf. The U.S. said the drone had been flying over international waters.

The U.S. called that incident unacceptable, and vowed the U.S. military would continue to fly such missions and would protect its aircraft.

In December 2011, a U.S. drone equipped with stealth technology went down in eastern Iran and was captured by Iranian forces.

This week’s incident comes amid the continuing standoff between Iran and the United States and its allies over the Iranian nuclear program.

President Barack Obama said March 14 that Iran could be “a year or so” away from being able to develop a nuclear weapon.

He said the United States continues to keep all options, including possible military action, on the table to prevent the development of an Iranian atomic bomb.

Obama spoke in an interview with Israeli television, ahead of his trip next week to Israel, an ally of the United States and a rival of Iran.

Israel has also not ruled out taking military action to prevent Iran from becoming capable of making a nuclear weapon. Iran denies any effort to develop an atomic bomb.

“The president feels that he’s been very clear on this subject: Our red line is that we will not allow Iran to obtain a nuclear weapon," Obama adviser Ben Rhodes told reporters. "The president has made clear, publicly and privately, that we reject the policy of containment because of the consequences of a nuclear-armed Iran for Israel, for the region, for the nonproliferation regime, and for the world."

Rhodes said Obama has "made clear that we will do what we must to prevent Iran from obtaining a nuclear weapon, and that means we’re looking at all options, including military options.”

Over the past year, the United States has boosted its naval and air force presence in the Gulf to help protect the Strait of Hormuz, the Persian Gulf waterway on Iran's southern coast which is used to transport some one-third of the world's seaborne oil exports.

Iran has threatened to blockade the strait in retaliation for sanctions targeting the Islamic republic over its nuclear program, human rights abuses, and alleged support for terrorism.